National Highways: Low Nutrient Grasslands - reducing maintenance and increasing biodiversity

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By Ben Hewlett, Senior Environmental Advisor – Safety, Engineering & Standards MSc, BSc (Hons)

With progress things are going to be built, it is inevitable, but if development can intelligently build in biodiversity - as a consideration from the outset and include in the planning stage then we might be onto a winner.

National Highways is a government-owned company charged with operating, maintaining and improving motorways and major A roads in England which is otherwise known as the Strategic Road Network (the SRN).

But our work goes beyond operating, maintaining and improving the 4300 miles of roads on the SRN and part of my role is to identify ways to improve them for biodiversity while still functioning as a piece of infrastructure.

A good example of this practice is a line of investigation around the soils we are using on our projects, which lead to the creation of a new policy for our Major Projects schemes – the National Highways Low Nutrient Verge Policy.

This policy, which was introduced in December 2020, specified that new areas of grassland that are created as part of a major project would no longer be covered in topsoil, leaving them as either bare substrate (chalk, sands, clays etc.) or bare subsoil and either left to naturally regen or seeded with appropriate wildflower seeds for the geology of the site.

The logic behind this being that the topsoil layer of the soil profile is typically where most of the organic matter and soil nutrients required for vegetation growth (nitrogen, potassium, phosphorus) exist.

However, the problem is that high soil nutrients creates a very comfortable environment for vegetation to establish and growth. This easy-going environment favours plants that have reproductive strategies to grow large very quickly, crowding other plants to become dominant. Think of brambles, stinging nettles, cow parsleys, thistles, docks etc. Without frequent management open areas of grassland can quickly become choked out by these species leading to a dominance of a handful of species when previously there may have been 10s/100s of species of plant per m2.

Although this vegetation forms its own habitat which will benefit certain species, this situation is an overall reduction of biodiversity. These sorts of sites can be appropriately managed to bring back grassland biodiversity, but it is an expensive process and time consuming - not always safe or practical to be carried out in a roadside environment.

This link with biodiversity and nutrient levels is everywhere once you start looking for it. All the old industrial sites that are now Sites of Special Scientific Interest (SSSI), brownfield areas, species rich grasslands on chalk or limestone with thin soils, heathlands on bare sands, ponds and rivers that have been negatively affected by an overabundance of nutrients needing leading to dominance of algae are a few examples. Even the Amazon rainforests soils are nutrient poor.

It is widely accepted that low nutrients in an ecological system leads to higher biodiversity. So, why when building do we put the most nutrient dense part of the soil profile back onto the verge once are finished building just to then struggle to manage its super charged vegetation forever? This is even before you think about how all the weed seed is in the topsoil layer that we are moving around sites. It didn’t make sense to me and thus the Low Nutrient Verge policy was born!

18 months on since National Highways introduced its Low Nutrient Verge policy has any progress been made? The short answer is yes. However, it hasn’t yet rolled out to other parts of the business like Operations Directorate.

In the world of Major Projects it takes a long time for policy change due to the length of time these projects take to plan and deliver (over many years). However, there are examples where this approach is being applied. The A2 Bean to Ebbsfleet improvements in Kent (*see further info below) is landscaped primarily with excess site soil won subsoil and are going to be seeded later this year. And there are lot more stretches of road in the pipeline M25 J10, M25 J28, M3 J9 which have plans to all include this approach across aspects of their design. So, good progress is being made.

The approach has also organically appeared in our Operations Directorate, which is the part of National Highways which is responsible for the day to day operation and maintenance of the road network.

For example, the A27 Patcham roundabout outside Brighton was a typical roundabout with shrubs in the middle which were having to be cut back twice a year due to impaired visibility for road users. A small project was undertaken to remove the vegetation and topsoil and replace it with crushed chalk which was then left to naturally regenerate. This work was carried out in September 2019 and since then the vegetation growth has been limited by the reduced soil nutrients.

The outcome is that the ground management bill, which previously was around £4K per annum, has reduced to almost zero and the capital project should pay for itself in 5 years. Looking ahead, over time more species will colonise the bare chalk, increasing biodiversity considerably more than what was there before.

So, a small intervention like this can save money, improve safety, and improve biodiversity. The question is how many more places like this are there on the network which would benefit from such an intervention?

There is a lot of opportunity, but hesitation still exists within the Construction Sector. My role is to highlighted the positives of considering biodiversity and illustrating good practice and the benefits where this has worked, from both a practical perspective and benefit to nature, as well as potential cost savings.

Currently, I am working with a project management team that was responsible fordelivering a drainage scheme which through its construction has exposed approximately 600m of bare chalk.

When I initially raised the idea of not topsoiling and direct seeding the chalk they struggled with the idea due to potential visual impacts. However, once reassured with other examples of colonisation and potentially significant cost savings from not having to import topsoil as previously planned (in the 10s of thousands) I appear to have won the team over. But how many other countless opportunities must there be in across the section from a national perspective to create the perfect conditions for wildflowers and save money?

I’ll finish on a positive note.

The Weymouth Relief Road shows the art of the possible with lack of topsoil. When constructed, the cuttings alongside the road were left bare seeded with chalk pioneer species, to ensure quick coverage of the chalk. There were concerns around the visual impacts of the cuttings as the site was in an Area of Outstanding Natural Beauty (AONB) and part of the main route to Weymouth/Dorchester used by both local residents, tourists and passing visitors.

If you visit or drive the road now it can clearly be seen that good vegetation coverage has been achieved. Furthermore, these cuttings also now happen to be one of the best places in Dorset for butterflies due to the abundance of wildflowers. In fact, half of all UK species of butterfly can be found there – all achieved in 10 years.

Links to further information:

• The A2 Bean to Ebbsfleet improvements in Kent (see last page)  
• Initiative launch press release 
  

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